He said: "It may be too late. If it's a radioactive poison with a short half-life it may have gone.

"Radioactive thallium degrades very rapidly so that by now we've missed the chance [to trace the poison]."

Radioactive thallium is used in hospitals, but Dr Henry said it was not used in massive doses consistent with Mr Litvinenko's condition.

'New dimension'

"Poisons can be taken by mouth, they can be injected, they can be inhaled," he said.

"In this case his symptoms are gastro-intestinal so the probability is that he has swallowed something that is poisoned.

"Radioactive thallium adds a new dimension to this case. It means that his bone marrow is at very high risk and we have to see how his cells recover. It is very difficult to treat because you have to rely on the body's natural resilience."

Mr Litvinenko's friend Alexander Goldfarb said: "He is even more tired than he was yesterday, but other than that - no change."

Prof David Coggon, Professor Of Occupational And Environmental Medicine said: "In general, the chemical toxicity would be no different for radioactive as compared with non-radioactive thallium.

"The former will, however, also pose an additional hazard from its radioactivity.

"The nature and risk of adverse effects from radioactivity will depend on where the thallium gets to in the body, in what quantities, how long it stays there, and what type of radiation is produced."

The critic of Russian President Vladimir Putin fell ill on 1 November after a meeting at a London sushi bar.

'Sheer nonsense'

Scotland Yard anti-terrorist officers have taken charge of the investigation into the poisoning by thallium of a former KGB colonel living in the UK.

The hospital said his condition was unchanged overnight.

The Kremlin has dismissed as "sheer nonsense" claims it was involved.

Friends of Mr Litvinenko have alleged he was poisoned because he was critical of the Russian government.

We don't consider it possible to comment on the statements accusing the Kremlin because it is nothing but sheer nonsense

Kremlin spokesman Dmitry Peskov said: "We cannot comment on the very fact of what happened to Litvinenko.

"We don't consider it possible to comment on the statements accusing the Kremlin because it is nothing but sheer nonsense."

Russia's foreign intelligence service has issued a statement denying any involvement.

Investigating murder

In a statement on Tuesday, University College Hospital said: "Mr Litvinenko's condition remains unchanged from yesterday. He remains in a serious condition in intensive care."

Mr Litvinenko had been investigating the murder of Russian journalist Anna Politkovskaya, an outspoken critic of Mr Putin and Russian policy in Chechnya, who was shot dead at her Moscow apartment building last month.

Mr Litvinenko fled to the UK in 2000 and was granted asylum

Doctors had told Mr Litvinenko he had a 50/50 chance of surviving the next three to four weeks, said Alex Goldfarb, who has been visiting him.

Scotland Yard said it was treating Mr Litvinenko's illness as a suspected "deliberate poisoning".

Officers are interviewing possible witnesses, including Mr Litvinenko, and examining CCTV footage, it said in a statement.